Optical frequency-division multiplexed (FDM) transmission systems as well as certain optical heterodyne processing functions desirably require the effective comb generation of optical frequencies from a single optical source. By generating optical signals each with a carefully defined frequency and offset, it is possible to transmit over a single-mode fiber a number of closely-spaced channels in order to exploit the fiber's enormous bandwidth potential. See, for example, K. Nosu et al., Journal of Lightwave Technology, Vol. LT-5, No. 9 pp. 1301-8(1987). Additionally, optical heterodyne detection, which is useful for selecting a signal among many distributed optical signals or many narrowly-spaced optically multiplexed signals, requires that the optical carrier wave frequency deviation be controlled to within approximately ten percent of the signal clock frequency. As such, it is extremely advantageous for the respective optical signals, which are offset by a definite frequency, to be synthesized from a single common optical source or "seed" so as to obviate the need for complex frequency stablization techniques.
In the prior art, various techniques have been proposed for generating an optical frequency comb, that is, a periodic set of optical signals having a fixed frequency offset. Such techniques have either been based on combining the output from many different optical frequency sources or modulating a single laser source vis-a-vis Mach-Zehnder based electro-optic frequency translators in order to produce a plurality of modulation sidebands. See W. A. Stallard et al., British Telecom Technology Journal, Vol. 4, No. 4 pp. 16-22(1986). Typically in Mach-Zehnder based translators, an optical frequency comb is generated by sinusoidally modulating an interferometer consisting of two Y-junction waveguide structures connected by LiNbO.sub.3 phase modulators. Splitting the incident optical signal into two paths with a phase modulation therebetween produces at the recombined Y-junction output optical signals that are generated at integer multiple frequencies of the incident light.
Although the prior art optical frequency comb generators perform acceptably, it is highly desirable not only to increase the intensity uniformity of the light at the translated frequencies, but also to provide a greater degree of flexibility and control in the characteristics of the optical frequency comb.